^ Depends on your area and brand of AR of course. $899 for an AR with a free-float rail and other options may not be too bad of a price, depending on quality and particular brand of rail/forend.
I've never heard of the R15 and just looked it up. looks like its more of a hunting-oriented AR. As long as you like it and the price is good to you, I don't see why not, if it fits your intended purpose. MSRP on Remington's site is $1200+ so $900 is actually not bad.
There are definitely other options out there at the same price range or even less.